Evolution of the lithospheric mantle under Boulaye volcano inferred from ultramafic lherzolite xenoliths (Adamawa plateau, Cameroon, Central Africa)

Abstract

This paper presents new data on mineral chemistry of ultramafic xenoliths from the Boulaye volcanic area, Adamawa plateau in Central Africa. Modal composition (olivine + orthopyroxene + clinopyroxene + spinel + amphibole) indicates spinel lherzolite with protogranular and porphyroclastic textures. Electron microprobe mineral analyses show that olivine is high Fo with variable CaO contents, orthopyroxene is calcium enstatite, clinopyroxene is Cr-diopside and spinel is Cr-spinel. Variations of contents in some major elements exhibited by olivine, pyroxene and spinel denote increasingly large degrees of melt depletion, emphasizing the complex tectono-magmatic history of the Adamawa lithosphere mantle. Xenoliths are equilibrated between 1027 and 1102 °C within pressure range of 0.9 to 1.7 GPa at equilibrium depths of 30 to 56 km. Boulaye xenoliths stand as mantle residues whose evolution is governed by melt extraction process (5 to 20%) overprinted by Ti–rich metasomatic fluid and/or melt. The geodynamic settings involve a Pan-African subduction event prior to upwelling of Adamawa lithospheric mantle, due to Pan-African strike-slip fault activity at Tertiary times.